浙东晚中生代玄武岩的锆石SHRIMP U-Pb年代学及其地质意义

SHRIMP U-Pb Dating of Zircons from the Late Mesozoic Basalts in Eastern Zhejiang Province and Its Geological Significance

对浙江东部晚中生代分属上下岩系的8个火山-沉积岩地层的玄武岩样品中的锆石进行了内部结构研究,并用SHRIMP方法测定了U-Pb年龄。结果显示,下岩系玄武岩中多数锆石给出206Pb/238U年龄为(120±1)Ma,Th/U比值为0.65～2.44;上岩系玄武岩中最年轻锆石的206Pb/238U年龄为(103±2)Ma,Th/U范围是0.57～3.03。上下岩系的形成时代的结果与前人研究的相应地层年代一致,但也存在有明显差别。在测定过程中发现,下岩系玄武岩中锆石年龄分布相对集中,仅有少量的古老锆石残留,表明在玄武岩形成过程中,只有少量地壳成分混染。上岩系中与玄武岩同时形成的锆石含量较少,多数是捕获锆石,且年龄分布范围较大,几乎覆盖了华南地块所有岩浆活动时代,这表明在玄武岩形成过程中捕获了陆壳物质,这种陆壳物质很可能是含有不同时期锆石的碎屑沉积岩/物。

The zircon U-Pb ages were dated by SHRIMP method for eight basaltic rocks occurred in the eastern area of Zhejiang Province,which were erupted during the Late Mesozoic and named as Lower and Upper rock series（LRSURS）.The weighted average 206 Pb/238 U age of the basalts from LRS is（120±1）Ma with Th/U ratios ranging from 0.65 to 2.44.The weighted average 206 Pb/238 U age of the basalts from URS is（103±2）Ma,with Th/U ratios ranging from 0.57 to 3.03.These data suggest that the zircons from both rock series are of magmatic origin and represent the formation times of the basalts of LRS and URS.Thus,it can be concluded that the formation time of the URS and LRS is significantly different,and the age data are consistent with the previous researches.It can also be found that most of the zircons from the LRS basalts have a relatively homogeneous ages with only a few ancient zircon relicts,which indicates that they have little crustal contamination during petrogenesis.Some zircon grains show core-rim structure with a core age of about 132 Ma and a rim age of about 120 Ma,which may indicate that the inherited grains were captured from the magma source,and imply that there exist magmatic rocks formed at about 130 Ma in the mantle or mantle-crustal boundary during the LRS magma activity.The magmatic zircons from URS basalts are very few,many detrital grains were dated with the ages covering almost all of the magmatic events in the South China block.It is suggested that the URS basaltic rocks were contaminated by clastic rocks which contained the detrital grains with different ages.